Touch up codes for UDP

* Move to `std::net` as it's all purely UDP related
* Rename to `UdpCodec` and `UdpFramed` to give a consistent `Udp` prefix
* Add `RecvDgram`, rename `SendDGram` to `SendDgram`
* Touch up some style here and there

src/net/mod.rs

@@ -8,4 +8,4 @@ mod udp;
 
 pub use self::tcp::{TcpStream, TcpStreamNew};
 pub use self::tcp::{TcpListener, Incoming};
-pub use self::udp::UdpSocket;
+pub use self::udp::{UdpSocket, UdpCodec, UdpFramed};